Much of this will depend on your listening style. If you tend to have a single mix that you keep randomized and repeat, a portable is a good match to that. There would be little interaction with the UI, and your capacity would depend on the type of player you buy.

For many people here, the ability to create extremely complex playlists, custom filters and nearly unlimited storage space does not allow a portable to be an option. If you are looking for playlists, searching, direct access to your music on any number of different criteria, an empeg is a better choice.

I have never used an iPod. There are some portable players that do have a number of very strong features and UI capabilities to them - such as the Karma. If you are looking to directly interface with a radio, you are pretty much limited to icelink and ipod, though. If you are going through an aux-in, you have more options, including an empeg mounted elsewhere in the car.

No matter what portable you use, though, unless it has extremely tight integration into the car head unit (think ipod and icelink only), any time you want to change the tracks, pause, etc., you will be "fishing" for the buttons on the portable as it wanders around your dash / console / seat. This is a really bad combination with driving! Make sure whatever solution you select has a good remote control option.

That is another reason the empeg is such a strong choice (admittedly from a biased user) - it is designed for in car use and the interface can be customized to your listening style.